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Light weights for large gains


Here is a brief summary:

  1. New research regarding the growth of Type I and Type II muscle fibers suggests that we have been neglecting our slow-contracting muscle fibers by training heavy.
  2. Type I muscle fibers are maximally stimulated by longer duration sets that require lower weights. Type II muscle fibers respond best to short sets with heavy weights.
  3. There are a lot of ways to vary the intensity in your training program, which include things like periodizing repetition ranges over time, as well as using heavier weights for multi-joint exercises and using lighter weights for isolation exercises.

"Train heavy to grow" is a favorite mantra among trainers and exercisers alike. Heavy weights maximally recruit large motor units associated with type II muscle fibers, and since type II fibers are the strength-related fibers that have the greatest growth potential, the key to maximizing muscle growth is to maximize their recruitment, right?

Well, not so fast...

Don't neglect your slow contracting muscle fibers

Type I fibers are like the Rodney Dangerfield of the bodybuilding world - they get no respect. Smaller, weaker and often smaller than their fast-contracting counterparts, Type I fibers are only famous for their ability to contract repeatedly - albeit without much force.

Relegated to a life of 5000 meter runs, marathons and disturbingly tight running shorts, the ability of these fibers to withstand fatigue seems to be more of a bodybuilding curse than a blessing. For this reason, bodybuilding training philosophies typically revolve around stimulating and exhausting Type II muscle fibers, while the slow-contracting muscle fibers don't get much attention.

However, new research on the effects of different training intensities and the growth of type I and type II fibers suggests that we have been doing the slow contracting muscle fibers an injustice and missing out on several kilos of potential muscle mass (1).

It's time to rethink our training philosophies in the context of fiber type specific hypertrophy.

Heavy weights and type II fibers

Certainly a large number of studies suggest that Type II fibers do indeed grow more with high-intensity strength training (2). The caveat here is the high intensity. It is not necessarily the case that type II fibers have an innate ability to outperform their slow-contracting cousins in terms of growth, but merely that they show superior growth when trained at high intensities (>50% of 1RM weight).

Our current understanding of hypertrophy of each fiber type may be more a consequence of the way it has been studied (high intensity) than of what actually happens in the gym (2, 3). The best summary of this relationship is a 2004 paper by Dr. Andrew Fry that summarized data from different studies regarding the growth rate of muscle fiber types and concluded that Type II fibers show superior growth at most exercise intensities.

However, when exercise intensity dropped below 50% of 1RM, type I fibers grew more than type II fibers, but growth in this range did not come close to what was achieved at higher intensities regardless of fiber type. After reading a study like this, not much would change in our training recommendations, but the type of analysis performed by Fry has its limitations (2).

The biggest limitation is that there have not been many low-intensity training studies to compare (2, 3) and there is a lack of studies that have directly compared high-intensity training to low-intensity training, taking into account the growth of different fibers.

Add to this recent research on the growth rates of muscle fibers in response to different training intensities (1) and you will quickly see that our type I fibers are capable of more than we have previously given them credit for.

A plea for Type I

Although they may be scarce, there are enough studies for us to conclude that we have probably underestimated the hypertrophy capacity of our type I fibers. Recently, Mitchell et al (1) conducted a now infamous training study that showed that training with low weights (three sets at 30% of 1RM), when performed to muscle failure, can produce comparable hypertrophy responses to training at higher intensities (three sets at 80% of 1RM).

While the data may not be statistically significant, when we look at individual fiber types, we see that Type I fibers are slightly more responsive to low-intensity training (19% change vs. 14%) and that Type II fibers are slightly more responsive to high-intensity training (15% vs. 12%).

This ultimately suggests that the equation involves more than the number of weight plates you put on the bar and tentatively supports what might be intuitively obvious: Type I muscle fibers are maximally stimulated by sets of longer duration with lower weights, while Type II fibers respond best to short sets with heavier weights.

A frequently criticized weakness in most training studies is that the scientists use untrained college students as test subjects. What happens in the untrained bodies of these subjects does not necessarily represent what happens in well-trained muscles. Fortunately, however, we also find support for our muscle fiber theories when we look at the muscle fibers of highly trained athletes.

Bodybuilders typically emphasize volume and fatigue and use moderate repetition ranges (4), while powerlifters (5) and Olympic weightlifters emphasize load and/or speed of movement. Not surprisingly, bodybuilders exhibit significantly greater hypertrophy of type I fibers compared to strength-oriented athletes (2).

Considering all the facts and evidence, it seems realistic to conclude that different training intensities can produce comparable overall muscle hypertrophy (1, 6-8), although the types of fibers involved may differ.

As with most things in the world of science, this is not a clear-cut issue. Two other studies with a slightly different study design have examined this issue and both of these studies concluded that high-intensity training is superior for growth regardless of fiber type (9, 10).

And this is where things start to get interesting. Although there are exceptions, studies where the work performed was the same at high and low intensity tended to favor high-intensity training for both fiber type-specific and overall muscle growth (10, 11). In the studies where the work performed at high and low intensity is not identical, equivalent results were observed at different intensities.

Ultimately, the idea that we have neglected the growth potential of type I fibers hinges on the argument that<

  1. hypertrophy requires a certain minimum time under tension, which varies depending on the training intensity and
  2. this time under tension is higher for type I muscle fibers than for type II muscle fibers.

Although no fiber type-specific effects were examined in this study, Burd et al. (12) compared the acute protein synthesis response to four training sets in three different scenarios:

  • 90% of 1RM to muscle failure
  • 30% of 1RM where the total work performed was the same as the 90% of 1RM scenario
  • 30% of the 1RM to muscle failure

Although the protein synthesis response varied slightly over time, it was quite similar in the two scenarios with training to muscle failure regardless of intensity. However, the protein synthesis response in the second scenario with an identical amount of work performed at 30% of 1RM weight - in which the time under tension was significantly less than in the 30% RM to muscle failure scenario - was only about half as strong as in the two scenarios with training to muscle failure.

Conclusion: Although the protein synthesis response to a single training session may not necessarily be an indicator of long-term adaptations, the fact that two studies show comparable hypertrophy when low-intensity training is performed to muscle failure provides further support for this idea (1, 6).

Does size matter?

The use of heavy weights is justified based on the fact that there is convincing evidence that these weights induce substantial hypertrophy independent of fiber type considerations (2, 9, 10, 13 - 17).

This is consistent with Hennemann's size principle of recruitment, which states that motor units are recruited in a specific order based on their size - smaller motor units are recruited in low force demand scenarios, while larger motor units come into play when force demands increase (18, 19). Heavy weights require more muscle mass to produce force, so you need to recruit more motor units from the start than if you were moving a light weight.

However, this argument does not explain the fact that fatigue could stimulate growth and that the onset of fatigue can directly influence the recruitment of motor units (20). If you move a relatively light weight, the recruitment of motor units at the beginning of the set will be lower than if you had started the set with a heavier weight.

Once fatigue sets in, you progressively recruit more fast motor units as the force-producing ability of the slow-contracting muscle fibers decreases (21). The size principle is maintained as you recruit motor units from the smallest to the largest, but you end up using fast contracting muscle fibers with a lighter load once you are fatigued.

This explains in part how the fast-twitch muscle fibers were able to grow during low-intensity training in the Mitchell et al. study (1) and why maximizing time under tension through exhaustion and muscle failure may be important in this concept.

Potential kilos of new muscle mass?

The idea that you are sacrificing kilos of muscle mass by ignoring training with lighter weights may seem like an exaggeration, but a brief overview of the fiber type composition of different muscles might change your mind.

Of course, the ratio of muscle fiber types can vary from person to person and is influenced by genetic predispositions and training (22), but considering that many of the major muscle groups have a substantial percentage of type I fibers - on average, people have roughly equal amounts of fast- and slow-contracting muscle fibers - this means that changing your training approach to optimize the growth of slow-contracting fibers may be worth a try.

Multiple repetition ranges are synonymous with maximum stimulation

For those looking to maximize their hypertrophy potential, it makes sense to cover the entire continuum of repetition ranges. While it may not be wrong to focus on the so-called "hypertrophy range" (6 to 12 repetitions), you should include both high repetition ranges (15 to 20+) and low repetition ranges (1 to 5) in your training program.

Such an approach not only ensures complete stimulation of the entire spectrum of muscle fibers, but also serves as preparatory work for optimal hypertrophy performance. Training with low repetitions promotes the neuromuscular adaptations necessary to develop maximal strength, allowing heavier weights (and therefore higher mechanical tension) to be used at moderate training intensities.

Performing sets with higher repetitions, on the other hand, can increase the lactate threshold over time, delaying the onset of fatigue and thereby increasing the time under tension when training with moderate repetitions.

There are countless ways in which varying intensities can be incorporated into a training program. Perhaps the best way to ensure continuous progress is to periodize the repetition ranges used during training. There are both linear and non-linear alternatives. It ultimately comes down to personal preference and individual goals (e.g. whether you are trying to maximize performance in a specific event).

Another option is to base load strategies on the type of exercise being performed. For example, you might choose to focus on low to moderate repetition ranges (~1 to 10) for multi-joint exercises such as squats, rows and presses, while prioritizing training with higher repetition counts (15+) for isolation exercises that are better suited to lighter training weights.

There are no hard and fast rules here. Responses to training vary from person to person and ultimately you will need to experiment with different approaches to find what works best for you.

Is there any hurry?

Type II fibers may beat Type I fibers in terms of hypertrophy superiority, but are you willing to take the risk of underestimating the potential of Type I fibers? Optimal hypertrophy training will give your fast-twitch muscle fibers the heavy weights they crave, while giving your Type I fibers the extended time under tension they deserve.

Note: Dan Ogborn, PhD, CSCS contributed to this article.


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By Brad Schoenfeld, PhD

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